1. Tissue models are necessary to estimate attenuation and acoustic exposure levels in situ from measurements of acoustic output made in water. Presently available models are limited in their ability to represent clinical conditions because of varying tissue paths during diagnostic ultrasound exposures and uncertainties in acoustical properties of soft tissues. No single tissue model is adequate for predicting in vivo exposures in all situations from measurements made in water, and continued improvement and verification of these models are necessary for making improved exposure assessments for specific applications.
2. A homogeneous tissue model with an attenuation coefficient of 0.3 dB/cm-MHz throughout the beam path is commonly used when estimating exposure levels. The model is conservative in that it overestimates the in situ acoustic exposure when the path between the transducer and the site of interest is composed entirely of soft tissue with or without bone.
3. For specific applications, a different homogeneous attenuation coefficient and a different power law frequency dependence would be appropriate to allow a more realistic safety and effectiveness analysis. For example, for normal breast imaging, an attenuation coefficient of 0.75 dB/cm-MHz1.5 [where the loss is
α z(dB) = α0(dB/cm-MHz1.5) f1.5(MHz1.5) z(cm)]
would be appropriate.
4. When the path contains significant amounts of fluid, as in many first- and second-trimester pregnancies scanned transabdominally, this homogeneous model may underestimate the in situ acoustical exposure. The amount of underestimation depends on each specific situation.
5. For these underestimated cases, “fixed-path” tissue models, in which thicknesses of soft tissue layers are held constant, are used to estimate in situ acoustical exposures when the beam path is greater than 3 cm and consists largely of fluid with negligible loss. When this model is used to estimate maximum exposure to the fetus during transabdominal scans, a value of 1 dB/MHz of the form
αz(dB) = α0(dB/MHz) f(MHz)
may be used as a conservative estimate of total attenuation during all trimesters.
6. More patient-specific models are becoming available, and their advancement is encouraged. More accurate acoustic characterization of tissues can be obtained to improve safety and efficacy.
7. Existing tissue models can be adjusted to accommodate attenuation, which may occur when significant nonlinear acoustic distortion is present.
